I do not suggest any tube rolling with Herron preamps. Keith cannily designs and voices his preamps using the most reliable tubes he can find - be it his superb engineering, voodo or black magic the gear always sounded best to me with the stock tubes. I wouldn't mess with success.
I haven't heard the 3 but I did the 2; I owned the upgraded version of Keith's first tubed preamp (the piece that put him on the map) and it was absolutely wonderful. His gear doesn't get the internet chatter that other amps do but all his products are wonderfully musical and huge bang for the $$s.
I could never really describe the "sound" of Keiths pre's, or even his power amps, so I always just say they "sound" like music, meaning they just let more of the music, and the musical event, through. "Lifelike" or "natural" also accurately describes the Herron house sound.
I feel the Herron amps are WAY underrated - they are just as terrific as the preamps. They are as close to big tube amps with bass slam as you will get for under 15k.
Sflazor, FYI one of the best sounding systems I've ever heard partnered Herron amps on ProAcs (M150s).
Interesting you say the Herron 3 preamp is sensitive to power cords - I tried many many high end preamps before I bought the Herron and that model. Whether it was due to the star grounding or wsomething else in the design implementation, was the least sensitive to power cords of any preamp I auditioned (CAT, First Sound, Arc, Joule, Blue Circle). This was true of the amps as well. The CAT< Joule and FS were far more sensitive to PCs.